[PATCH v2 39/51] powerpc/crash_dump: recover from a machine check in copy_oldmem_page()

[Christian Brauner]

A capture kernel reads the memory of the crashed kernel, and that memory
may well have uncorrectable errors. On powerpc a machine check in the
kernel is only recovered from if the instruction that took it has an
exception table entry (cf. mce_common_process_ue()). copy_oldmem_page()
copies with copy_to_iter(). For a read() of /proc/vmcore that is a user
copy, all of whose loads have exception table entries, so the machine
check turns into a short copy and read_from_oldmem() returns -EFAULT.

But since commit 4a22fd20379c ("vmcore: convert __read_vmcore to use an
iov_iter") /proc/vmcore can be spliced too, and splice(), sendfile()
and copy_file_range() copy into kernel pages with memcpy(), which has
no exception table entries. A machine check there panics the capture
kernel. The same goes for the reads of the ELF headers and notes that
vmcore does itself, and those have been memcpy() for as long as
copy_oldmem_page() has had its kernel buffer branch, since commit
54c32021eb6f ("[PATCH] powerpc: Add arch-dependent copy_oldmem_page").

Copy with _copy_mc_to_iter() instead. PPC64 selects ARCH_HAS_COPY_MC
and there it uses copy_mc_generic() for kernel and user destinations
alike, which has the exception table entries. Everywhere else it is
_copy_to_iter().

Reported-by: Chris Mason <mason@xxxxxxxxxx>
Fixes: 54c32021eb6f ("[PATCH] powerpc: Add arch-dependent copy_oldmem_page")
Fixes: 4a22fd20379c ("vmcore: convert __read_vmcore to use an iov_iter")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Christian Brauner (Amutable) <brauner@xxxxxxxxxx>
---
 arch/powerpc/kernel/crash_dump.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/crash_dump.c b/arch/powerpc/kernel/crash_dump.c
index 1543917bd3e1..ca7e019ae3e9 100644
--- a/arch/powerpc/kernel/crash_dump.c
+++ b/arch/powerpc/kernel/crash_dump.c
@@ -83,12 +83,12 @@ ssize_t copy_oldmem_page(struct iov_iter *iter, unsigned long pfn,
 
 	if (memblock_is_region_memory(paddr, csize)) {
 		vaddr = __va(paddr);
-		csize = copy_to_iter(vaddr + offset, csize, iter);
+		csize = _copy_mc_to_iter(vaddr + offset, csize, iter);
 	} else {
 		vaddr = ioremap_cache(paddr, PAGE_SIZE);
 		if (!vaddr)
 			return -ENOMEM;
-		csize = copy_to_iter(vaddr + offset, csize, iter);
+		csize = _copy_mc_to_iter(vaddr + offset, csize, iter);
 		iounmap(vaddr);
 	}
 

-- 
2.53.0